Tear biomarkers in veterinary species: An updated review with special focus on dogs and cats.

The tear film is traditionally described as a highly organized, multilayer structure comprising an outer lipid layer, a central aqueous phase, and an inner mucin layer, each contributing distinct structural and functional properties. Beyond its essential roles in maintaining ocular surface homeostasis, tear fluid represents an integrated biological secretion shaped by systemic physiological regulation that might be used as a source of biomarkers. While tear fluid has long been recognized in human medicine as a valuable reservoir of biomarkers, its diagnostic and translational potential in veterinary medicine has only recently begun to emerge. This narrative review provides an updated and comprehensive overview of the physiological characteristics and biochemical composition of tear fluid. It covers how tear composition changes under pathological conditions and outlines the most widely used sampling techniques-such as Schirmer strips, capillary tubes, and absorbent materials-discussing their advantages, limitations, and their influence on sample quality. It further explores the identification of tear biomarkers and significantly expanded the diagnostic potential of tear-based research in animals. This will contribute to a better knowledge as tears as a source of biomarkers that represents a promising, non-invasive medium for the detection and monitoring of both ocular and systemic diseases in animals.